Making nestjs-slack a pure ESM package

[simenandre]

While there is soon to be a breaking change (moving this package to v2), I think it's time to discuss when to move to pure ESM.

I know there are a lot of teams that still use CommonJS, and from what I can understand the nest community is slowly moving that way. This means we can start planning for a new major release (v3).

Feel free to add feedback on this, and maybe share a timeline for when you are going ESM! I think we will keep maintaining v2 for quite some time, regardless of when v3 comes out.

Thanks ❤️